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  High performance light-emitting devicesUSPTO Application #: 20070246705Title: High performance light-emitting devices Abstract: An organic light emitting device consists of a layered structure including a top multilayer stack, a bottom multilayer stack, a cavity layer between the top multilayer stack and the bottom multilayer stack, and an organic light emitting region within the cavity layer. The layered structure is constructed such that the product of phase factors ξ1 and ξ2 is. greater than 80% at the center of at least one emitting wavelength region and for a normal viewing angle, wherein where Ra− and Rb+ are the reflectance of the top and bottom multilayer stacks respectively, φa− and φb+ are the phase changes on reflection for the top and bottom multilayer stacks respectively, α1 β1 are respectively the real and imaginary parts of the phase thickness of the cavity layer, α2 and β2 are respectively the real and imaginary parts of the phase thickness of the light-emitting region at the operating wavelength of the device, x is the mean distance of light emitting region from the bottom multilayer stack, n and k are the refractive index and absorption coefficient of the cavity layer, θcavity is the emitting angle inside the cavity layer, and d is the physical thickness of said cavity layer. This condition improves the light output efficiency of the device. (end of abstract) Agent: Marks & Clerk - Ottawa, ON, CA Inventors: Li Li, Jerzy Dobrowolski, Daniel Poitras USPTO Applicaton #: 20070246705 - Class: 257040000 (USPTO) Related Patent Categories: Active Solid-state Devices (e.g., Transistors, Solid-state Diodes), Organic Semiconductor Material The Patent Description & Claims data below is from USPTO Patent Application 20070246705. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD THE INVENTION [0001] This invention relates to the field of light emitting devices, and more particularly to light emitting diodes or displays, wherein a light-emitting layer is sandwiched between top and bottom multilayer stacks. The invention is primarily, but not necessarily exclusively applicable to OLEDs (Organic Light Emitting Diodes). BACKGROUND OF THE INVENTION [0002] Unlike liquid crystal displays (LCDs), OLEDs are emissive displays and do not require backlighting. They are made of mostly organic materials and thus have the advantage of potentially low manufacturing cost, full color capability, light weight and flat structure. They can be used as flat panel displays in many applications such as computer monitors, personal assistant devices, automobile displays, etc. [0003] A basic OLED device as shown in FIG. 1 consists of a transparent substrate 16 made of glass or plastic, a transparent anode layer 21 such as ITO, an organic hole transport layer (HTL) 20, such as N,N'-diphenyl-N,N'-bis(3-methylphenyl)(1,1'-biphenyl)-4,4'-diamine (TPD) or N,N'-bis(I-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB), a light emitting organic layer 10, such as tris(8-hydroxyquinolinato)aluminum (Alq.sub.3), which is also an electron transport (ETL) layer, a cathode 23 made of metal or metal alloys, such as Al, Mg, Mg:Ag and an optional cover glass 24. When a voltage is applied between the cathode and the anode, electrons and holes are injected respectively from the cathode into the ETL layer and from the anode into the HTL layer. The injected electrons and holes migrate towards each other because of the electrical potential between the anode and the cathode, and recombine inside the ETL layer near the interface between the ETL and HTL layers. As a result, energy is released in the form of electroluminescent light, which exits through the transparent substrate. The color of the emitted light is determined by the energy band gap of the emitting layer. In the case of Alq.sub.3, usually green color light centered at 530 nm is emitted. Different colors of emitting light can be obtained by using different organic emitting layers, or by doping the same organic emitting layer with different color dyes. White emitting light can be achieved by doping the emitting layer with red (R), green (G) and blue (B) dyes or by stacking R, G, and B emitting layers on top of each other. Full color displays can be realized in several approaches as described in detail in a paper by Burrows et al. in the IEEE Transactions on Electron Devices, Vol. 44, No. 8, 1997, p. 1188-1203. Among these approaches, using three color pixels, red (R), green (G) and blue (B) to form a full color pixel is rather simple and has been commonly used in other direct view flat panel displays such as LCDs. [0004] Unlike thin film electroluminescent displays (TFELs), OLEDs are current-driven devices. The amount of light emitted is directly linked to how much current is passing through the device. Usually, the higher the current, the more light is emitted and the brighter the displays are. Unfortunately, two major problems with current OLED devices are their short lifetime and poor stability, problems that are both directly linked to the driving current. The performance of an OLED degrades quickly when it is driven at a high current level and so does its lifetime. Thus, if the efficiency of the OLEDs can be enhanced, then the driving current can be reduced, and the lifetime and stability of the OLED devices can be improved as well. [0005] The efficiency of the OLEDs is mainly affected by two factors: the internal quantum efficiency and the external extraction efficiency. The first factor indicates how many electrons or holes can be generated and how many of them can recombine and emit photons in the desired spectrum. This factor is determined by the choice of OLED materials and structures, such as the cathode, anode, electron and hole transport layers, etc. The external extraction efficiency indicates how much generated light exits the OLED structure. This factor is mainly determined by the optical thin film structure of the OLEDs, such as the refractive indices and layer thicknesses of all layers in the OLED structure. It is well known that an OLED structure resembles that of a Fabry-Perot microcavity that has been extensively used in solid-state light emitting diodes (LEDs) and vertical cavity surface emitting lasers (VCSELs). But unlike those in LEDs and VCSELs, the cavity effect in OLEDs is much weaker due to the low reflectance of the top layer structure including the hole transport layer and the anode. Many efforts have been made to improve the extraction efficiency of OLEDs by increasing the cavity effect using mirrors with higher reflectance. Such effort did result in the enhancement of the peak wavelength emission in OLEDs; unfortunately, this enhancement is often accompanied by the reduction of the emission at other wavelengths, the width of the emission wavelength band and the viewing angle. These side effects are not desired in display applications, which usually require viewing angles larger than .+-.60.degree. and an emission bandwidth covering the whole visible spectrum region (400-700 nm). [0006] Furthermore, the cathodes used in OLEDs are often made of metals or metal alloys having high reflectivity. They strongly reflect ambient light and as a result significantly reduce the contrast of OLED devices when they are viewed under strong ambient light illumination. Many methods have been suggested to improve the contrast of OLEDs devices. One approach is to use expensive circular polarizers that allow at most 37% of the emitted light to pass through. Another approach is to use black layers to reduce the reflectance of the cathodes. In particular, the thin film interference black layer approach, first disclosed in U.S. Pat. No. 5,049,780 by Dobrowolski et al. and successfully applied to thin film electroluminescent devices, has been recently applied to OLEDs in several US patents, for example, U.S. Pat. No. 6,411,019, U.S. Pat. No. 6,551,651, U.S. Pat. No. 6,429,451, and U.S. Pat. No. 6,608,333. If a perfect black layer with zero reflectance was used, then there would be no cavity effects at all, and the amount of light emitted from such an OLED device would be about four times less than that of a similar OLED device having a conventional cathode with a high reflectance. Furthermore, if the black coating is not perfect and has some residual reflectance, which is often the case in real life, the residual reflectance could result in a cavity effect. The emission can be either enhanced or reduced by this cavity effect, depending on the critical factors of the phase changes on reflection of the two mirror structures. The phase changes on reflection are not considered in the above US patents. In addition, such interference black layer coatings described in these patents require the use of at least one transparent (or semi-transparent) layer and one absorbing layer, all inserted between the conventional metal cathode and the emitting layer. In OLEDs these added layers are required to be conductive and the layer next to the organic emitting layer is required to have a low work function (preferably <4 eV) in order to allow electrons to be injected easily into the emitting layer. The number of transparent conducting materials is limited and these often have high work functions (e.g. 4.4-4.8 eV for ITO) and thus are not suitable to be used next to the emitting layer on the cathode side. In addition, the fabrication of transparent, dense and high-conductivity materials usually requires high-temperature deposition, which is not acceptable when depositing on organic materials, most of them being temperature sensitive. Low-temperature deposited transparent materials have been used for the fabrication of interference black layer coated cathodes. In addition, semi-transparent metal/dielectric mixtures have been proposed to replace the transparent layer in the thin film interference black layers for OLED devices. Such replacements result in poorer black layer performance than those used in TFELs because their optical constants are less suitable for the design of lower-reflectance black layer coatings. Moreover, these semi-transparent layers are deposited by the simultaneous evaporation of metal and dielectric materials. The properties of these co-deposited layers, such as their conductivities and optical constants, vary greatly with the deposition rates of the two materials [Han et al. J. Appl. Phys. 96, 709 (2004)], and so the performance of the OLEDs incorporating such black layers will be also be affected. Furthermore, such black layer coatings are not effective in reducing the light reflected from other interfaces of the OLED devices. SUMMARY OF THE INVENTION [0007] According to the present invention there is provided a light emitting device comprising a layered structure comprising a top multilayer stack, a bottom multilayer stack, and a cavity layer between said top multilayer stack and said bottom multilayer stack; an organic light emitting region within said cavity layer; and wherein said layered structure is constructed such that the product of phase factors .xi..sub.1 and .xi..sub.2 is greater than 80% at the center of at least one emitting wavelength region and for a normal viewing angle, wherein .xi. 1 = ( 1 + 4 .times. R a - .times. R b + .times. sin 2 .function. ( .alpha. 1 - .phi. a - + .phi. b + 2 ) .times. e - 2 .times. .beta. 1 ( 1 - R a - .times. R b + .times. e - 2 .times. .beta. 1 ) 2 ) - 1 .xi. 2 = 1 - 4 .times. R b + .times. sin 2 .function. ( .alpha. 2 - .phi. b + 2 ) .times. e - 2 .times. .beta. 2 ( 1 + R b + .times. e - 2 .times. .beta. 2 ) 2 .times. .times. and where R.sub.a.sup.- and R.sub.b.sup.+ are the reflectance of the top and bottom multilayer stacks respectively, .phi..sub.a.sup.- and .phi..sub.b.sup.+ are the phase changes on reflection for the top and bottom multilayer stacks respectively, .alpha..sub.1 and .beta..sub.1 are respectively the real and imaginary parts of the phase thickness of said cavity layer, .alpha..sub.2 and .beta..sub.2 are respectively the real and imaginary parts of the phase thickness of said light-emitting region at the operating wavelength of the device, x is the mean distance of light emitting region from the bottom multilayer stack, n and k are the refractive index and absorption coefficient of said cavity layer, .theta..sub.cavity is the emitting angle inside the cavity layer, and d is the physical thickness of said cavity layer. [0008] The invention is primarily concerned with OLEDs, where the light emitting material is organic, but it would be applicable to any device of similar structure having a light emitting region within a layered structure. [0009] For any given OLED it will be possible to determine whether it satisfies this criterion. An OLED that satisfies this criterion will have enhanced properties relative to the prior art due to the improved efficiency of the light emission from the device. Prior art devices wherein the thickness of the layers was not calculated to optimize light efficiency had a product of phase factors .xi..sub.1 and .xi..sub.2 of less than 80%. Preferably, this product should be 90%, and optimally 95%, or more. It will be appreciated that although for convenience the OLEDs in accordance with the invention are described as having top and bottom multilayer stacks, they can be oriented in any manner. However, it will always be possible to orient a device in a manner consistent with the present disclosure. [0010] The invention permits high efficiency OLED devices to be obtained by correctly selecting the layer thickness of each layer of an OLED structure to maximize the external extraction efficiency without significantly affecting the internal quantum efficiency. [0011] Even higher efficiency OLED devices can be obtained in accordance with the invention by introducing additional thin film layers that take advantage of the microcavity enhancement effect and that at the same time minimize the common problems of the reduction of the angular field and of the bandwidth. [0012] In another embodiment high efficiency and high contrast OLED devices are obtained by introducing additional thin film layers that take advantage of the microcavity effect and which simultaneously minimize the reflectance not only of the cathode layer but also of the whole OLED structure and which also reduce the problem of the narrowing of the angular field and of the bandwidth. BRIEF DESCRIPTION OF THE DRAWINGS [0013] The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: [0014] FIG. 1 is a cross-sectional view of the basic structure of a conventional OLED; [0015] FIG. 2 is a cross-sectional view of the OLED structure in accordance with the present invention; [0016] FIG. 3a shows the calculated T.sub.emax value as a function of R.sub.a.sup.- for A.sub.a.sup.-=0.00 and for values of R.sub.b.sup.+ in the range of 0.80-0.99; [0017] FIG. 3b shows the calculated T.sub.emax value as a function of R.sub.a.sup.- for A.sub.a.sup.-=0.00 and for values of R.sub.b.sup.+ in the range of 0.00-0.25; [0018] FIG. 3c shows the calculated T.sub.emax value as a function of R.sub.a.sup.- for A.sub.a.sup.-=0.60 and for values of R.sub.b.sup.+ in the range of 0.80-0.95; [0019] FIG. 4a shows the calculated T.sub.emax value as a function of R.sub.a.sup.- for A.sub.a.sup.-=0.05 and for values of R.sub.b.sup.+ in the range of 0.80-0.99; [0020] FIG. 4b shows the calculated T.sub.emax value as a function of R.sub.a.sup.- for A.sub.a.sup.-=0.05 and for values of R.sub.b.sup.+ in the range of 0.00-0.25; Continue reading... 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